This invention relates generally to alarm devices which sense the occurrence of a problem and produce an alarm, particularly such alarm devices designed for home use. One such device that has attracted a great deal of interest is the smoke detector, which is battery powered so as to be self-contained. The detector has a horn which emits an alerting tone.
It has been determined that several such smoke detectors should be mounted at key locations in the home. For example, one might mount a smoke detector in the upstairs hallway, another in the kitchen, another in the basement, another in the garage, etc. The occurrence of smoke exceeding some predetermined intensity will set off the alarm in the location nearest the smoke. However, the occupants may be so far from that particular alarm device that they cannot hear it. For example, if the family is sleeping, they may not hear the horn in the smoke detector located in the basement.
It has, therefore, been proposed to couple in parallel the horns of the smoke detectors such that the activation of one by smoke will automatically energize the horns in all of the detectors. Following through on the above example, smoke in the basement will set off not only the basement smoke detector, but also the detector in the upstairs hallway, thereby alerting the occupants.
The difficulty with this approach is that if a short occurs across the interconnecting pair of wires, all of the horns are disabled, including the horn in the detector which had been set off. Furthermore, the shorted horn in each smoke detector places the entire battery voltage across the switching device which may, for example, be an SCR, thereby rendering it useless. Finally, one battery--the one in the activated smoke detector--must supply current for all the horns, thereby greatly reducing its useful life.